At present, joints between thermoplastic tubes and connections are performed by means of thermofusion joining, whereby during a few seconds tube and connection are subject to a temperature in the range of from 260 to 270° C. and after the elapsing of the heating time both elements are joined by introducing the male end of one of them into the female opening of the other, with an adequate interference grade between their facing superficial layers, and they are fused, i.e. they are transformed into a single piece through them.
It is highly inadvisable to join tubes bearing a metal layer to the connections thereof by means of thermofusion, as such joining would only be accomplished between the external thermoplastic layer and the connection wall, the intermediate metal layer edge thus remaining internally uncovered, whereby water circulating through tubing would produce an oxidising process with the consequent deterioration of the metal layer, which would inevitably result in water leaks and/or contamination.
On the other hand, even where an abutting supplementary union could be done between the internal layer of the tube and an internal cylindrical surface of such connection, a highly resistant section would not be possible to obtain due to the reduced thickness of said internal layer, which section could not absorb the shearing force which causes bending of the connection wall upon the expansion due to the pressure of fluid working at high pressure, which on the other hand is easily absorbed by the tube without any alteration whatsoever due to the presence of the intermediate metal layer and which would obviously produce breakage of this joint and the resulting water-metal contact.
Consequently, connection between tube and any class of connection requires from the later to be provided with a tubular pin which is tightly inserted into said tube end, around which an external snap-fastener is applied in order to tighten same around the pin, thus creating an airtight joint between the internal thermoplastic layer of the tube and said pin. Due to said tubular pin thickness, a noticeable reduction of the tubing internal diameter is produced, giving rise to several drawbacks.
Further, this kind of connection is very expensive, not only due to the manufacturing cost of the piece itself but also due to the tools and labour required for the application thereof.